How to Insulate Between Roof Rafters

Insulation between roof rafters

If you’re struggling to stay warm and your energy bills are skyrocketing, your home could probably use some insulation. You probably heard that you should start with insulating the attic. That’s true; homes lose up to 25% of heat through roof systems.

Insulation between roof rafters is one of the best ways to make your home more energy-efficient and comfortable. Installing insulation in this often-neglected area can be a game changer in managing the temperature of your living spaces below.

In this guide, we’ll explain how to insulate between roof rafters. So let’s get started.

What’s Rafter Insulation?

Rafter insulation refers to adding thermal insulation material between or over the wood beams supporting your roof. 

The main point behind rafter insulation is to reduce the heat transfer between your home’s interior and exterior. That way, your house stays warm in winter by retaining heat, while in summer, it stays cool by holding on to the cooler air in your home.

Is It Essential to Insulate Between Roof Rafters?

You might wonder if adding insulation to the top of your attic floor would be easier than insulating between the rafters. 

Well, insulating the attic floor is an excellent way to improve your home’s energy efficiency; it might be easier than insulating roof rafters. However, insulating between roof rafters has tremendous benefits, especially in the long run.

Think of it this way: your roof is the first layer of protection against external elements and covers a large surface area. It’s not just about energy efficiency; insulating rafters have other benefits, too. 

Let’s see how you could benefit from insulating between rafters.

1. Energy Efficiency

Insulating your roof rafters is one of the most effective ways to enhance your home’s energy efficiency. This ultimately results in lower energy bills and an overall more comfortable home. 

The insulation also acts as an extra layer of protection against external elements. This doesn’t only protect your home, but it improves the indoor air quality as well.

2. Moisture Control

Rafter insulation can make a world of difference in controlling moisture. That’s because it prevents the accumulation of moisture, which is the main cause of more serious issues like mould and rot. 

Accordingly, insulation improves your roof’s overall strength and durability and provides extra protection against fire events.

3. Environmental Impact

You might not think about it, but insulating between rafters can significantly decrease greenhouse gas emissions and preserve energy resources. But how?

Proper rafter insulation gives you more control over your house’s internal temperature, and the need for excessive heating or cooling is reduced. 

In other words, you don’t use your internal cooling and heating systems too much, which helps reduce carbon emissions into the atmosphere.

4. Increased House Value

Insulating between roof rafters doesn’t only save you hundreds of pounds on energy bills, it can increase your home’s value, too. 

Energy-efficient properties are always in high demand, and it’s something that potential buyers are looking for. So, even though some types of insulation can be expensive upfront, they’ll save you a lot of money in the long run.

5. Soundproofing

Another benefit of insulating between roof rafters is that it can reduce the noise coming into your home. Adding insulation between rafters can absorb sound waves, creating a quieter indoor environment. 

So, if the sound of rain and traffic bothers you, you could benefit greatly from insulation between your roof rafters.

UK Building Regulations for Insulation Between Roof Rafters

When it comes to installing insulation between roof rafters in the UK, there are specific building regulations you must adhere to. 

1. The U-Value

As you probably know, the required insulation level varies depending on the building construction, roof type, and climate zone. Additionally, to insulate between roof rafters, the U-value of your building element should be within a specific range.

If you have a pitched roof, the UK building laws require a U-value of 0.18 W/m2K, at least for a warm roof. Meanwhile, for a cold roof, the minimum U-value is 0.13 W/m2K. 

2. Recommended Insulation Thickness

The thickness of insulation material between rafters is a crucial factor in achieving an optimal level of thermal performance. 

That’s why the UK regulations state that the recommended thickness range should be between 170mm and 350mm. That depends on the type of insulation used and the desired level of thermal performance. 

However, the regulations specify a minimum thickness of 270mm for insulating new and existing pitched roofs. So, what about the relationship between the recommended thickness and insulation type?

Let’s say you’re using PIR insulation, which has a higher R-value than other insulation materials (around 4.0 m2K/W per 100mm). This means that even if you use a thin layer of PIR insulation, it will still perform as well as thick layers of other insulation materials.

On the other hand, 100mm fibreglass has a relatively low R-value of 2.25m2K/W. As such, if you increase the insulation thickness to 270mm, you can achieve an R-value of 6.9m2K/W.

3. Ventilation Requirements

If you’re building a fully ventilated roof, you must have a ventilated air gap joining the insulation and the roof covering. It should be around 50mm. This is mainly to avoid condensation.

On the other hand, if it’s an unventilated roof, you must ensure enough space for the breathable membrane to drape. However, this will reduce the available space for insulation.

How to Insulate Between Roof Rafters in Steps

Before you start installing insulation between roof rafters, there are some things you need to consider. 

1. Inspect the Attic and Rafters

If you can access your attic, insulating between roof rafters should be a relatively easy DIY job. But first, you need to carefully inspect the attic and between rafters. Look for gaps, leaks, or damages that might need to be repaired before installing the insulation.

If everything is fine, start by using a vacuum to clean the rafter space, as there’s always a lot of dust and debris in there. 

After that, make sure you lay some boards on the joists so that you can walk around the attic safely. 

2. Taking Measurements

After you ensure the area is clear of bulky objects and safe, it’s time to take measurements.

Start by measuring the rafter area using a tape or a laser measure. Measure from the highest point of the roof and down to where the sloping roof connects with the horizontal joists. Then, multiply the measurement by the roof’s width.

Remember to take the thickness you want to insulate into account. For optimal insulation, you need to insulate 100mm at least.

You might also need to measure the rafters’ width if you’re planning to insulate below them.

Finally, write down your measurements so you have an idea of how much insulation material you need to buy.

3. Decide Which Insulation Material to Use

There are various types of insulation materials out there. Your choice will depend on many factors, such as your needs and budget. Here are the most common insulation materials used between roof rafters.

1. Mineral Wool Insulation

Mineral wool is a popular choice for insulation between roof rafters. It’s easy to install, affordable and does a great job of reducing heat and noise transfer. It’s also a safe option as it’s non-combustible and is made of mostly organic materials.

2. Fibreglass Insulation

Fibreglass is another great choice for insulating between roof rafters. It’s cost-effective, lightweight, and has excellent thermal insulation properties. Be careful; fibreglass can irritate your skin and lungs, so you must wear protective gear and masks when handling it.

3. Phenolic insulation

Phenolic insulation is an excellent option when you have limited attic space. That’s because it provides efficient thermal insulation even when used in thin layers. It’s moisture and fire-resistant, making it a safe choice as well.

4. Polyisocyanurate (PIR) Insulation

As we previously mentioned, PIR insulation has a high R-value, meaning it provides a high level of thermal insulation in a thin profile. 

It’s one of the most commonly used materials for insulation between roof rafters, thanks to its convenience, availability, and affordability. On top of that, it’s lightweight and has moisture and fire-resistant properties, which is ideal for insulating between rafters.

4. Installing Mineral Wool Insulation

If you decide to use mineral wool as an insulation material, you must measure the space between rafters. 

The standard rafter spacing in older buildings in the UK is 400mm; meanwhile, in modern buildings, it’s 600mm. Knowing your measurements won’t only ensure that the mineral wool has a snug fit, but it will also minimise waste. 

Before installation, make sure you leave a space of at least 50mm between the insulating material and the roof to eliminate the risk of condensation and damp issues.

Some mineral wool types come in a standard size of 600x1200mm, so you might need to cut it using a blading saw or a sharp knife according to your measurements. 

You may feel like you’re wasting a lot of material at first. But even if there’s some material left after installation, it can always be reused for insulating other areas of your roof, so don’t worry.

We suggest cutting the insulation material 1cm wider than the gap between your rafters to allow for a snug fit. Start at the top and staple the mineral wool to the sides of your rafters. Continue fixing the material until the whole place is covered. 

Depending on the rafter’s thickness, you can fit two layers of 100mm mineral wool. After that, you can add a layer of netting for extra support against gravity. 

Note that sometimes you might need to compress the wool a bit to fit it into the gaps, but be careful not to over-compress it as this will reduce its effectiveness.

To improve the visuals of your roof, you can apply a layer of plasterboard to the rafters and paint over it. Although this might take up some space, it will make your roof much more attractive.

5. Installing Fibreglass Insulation 

Fibreglass insulation is usually sold as rolls with a width of 400mm and 600mm to match the standard rafter spacings of UK houses. 

First, you need to cut the rigid fibreglass boards to fit the gaps between your rafters. Make sure the edges fit snugly so that you can easily staple them to the rafters. 

The wooden battens in your rafters will serve as automatic stabilisers, allowing the boards to rest on top of them with ease. Next, secure the insulation in place using a wire or a string and then add a VCL membrane to avoid moisture issues.

One major difference between fibreglass insulation and mineral wool is that you have to be extra precise while dealing with fibreglass. That’s because the cuttings need to be almost perfect to avoid any gaps. On the other hand, you can easily mould and fold wool to fit snugly in your rafter space. 

Mineral wool could also use some compression to fit well into gaps. Meanwhile, you should never compress fibreglass because it loses its effectiveness. You should only fluff it up a bit after unrolling it from the roll.

6. Installing PIR Insulation

Rigid PIR insulation boards are among the best and most commonly used insulation materials when it comes to roof rafters. Let’s see how to install them.

Start by determining the size of the required insulation boards by measuring the space between your rafters. Then, cut the boards according to your measurements using a hand saw. 

We suggest trimming smaller pieces first and then making any necessary adjustments if needed. That way, you’ll get a snug fit to minimise the risk of air gaps and heat loss.

Once again, don’t forget to leave an air gap of at least 25mm or 50mm between the rafters and the PIR insulation boards to allow air circulation and prevent dampness and condensation problems.

Now, you should move on to fixing the insulation boards between the rafters. To ensure there are no gaps left, use wooden battens and tap the boards into place with a hammer. This ensures the boards are firmly in place and flat against the rafters, preventing any leaks or air gaps.

But what if the insulation boards are too tight to fit into the space? In that case, never try to force the boards between the rafters, as this will cause damage to the insulation. Instead, remove the boards and trim them down a bit so they fit better.

After you make sure the insulation boards are in place and tightly secured, inspect your rafters again for any gaps or leaks. If you see any cracks, fill them with low-expansion foam to improve the thermal insulation properties of your setup. 

But remember not to overfill the voids because compressing the insulation boards reduces their effectiveness. 

To keep your attic safe from moisture issues, it’s always wise to apply a vapour control layer on the warm side of your attic. This prevents any moisture from getting into the insulation and damaging the structure of your roof. 

When You Shouldn’t Insulate Between Roof Rafters

Even though insulating between roof rafters can make your home more eco-friendly and comfortable, there are some situations in which you should avoid this type of insulation.

1. Flat Roofs

If you have a flat roof, it’s not recommended to insulate between roof rafters. That’s mainly because flat roofs benefit from insulation installed on top of the roof deck, not between the rafters. 

Installing insulation between rafters in a flat roof can cause serious structural damage, such as moisture and mould buildup.

You may find other flat roof insulation to be more beneficial. 

2. Ventilated Attics

Ventilated attics rely on air circulation to maintain proper temperature control and prevent moisture buildup. 

So, if you insulate between roof rafters in a ventilated attic, you’re hindering the airflow, leading to potential damage to your roof.

3. Attics With Structural Issues

As we previously mentioned, checking your attic for existing structural problems is crucial before you start insulating between roof rafters. We’re not just talking about minor leaks and gaps here. 

Sometimes, there are more serious problems that need professional assistance to fix before moving on with the insulating process.

For instance, the rafters might not be adequately supported. If you proceed with rafter insulation in that case, the rafters might collapse and cause severe damage to your home.

Opt For Professional Installation Instead

Insulating between roof rafters might well be appropriate for DIY enthusiasts, but we recognise that most of you won’t be so keen on DIY tasks. That’s where Insulation Advisor comes in. 

We can compare local insulation installers near you who are all professional and vetted by us, meaning you know you can trust them. Compare insulation installers with us today and find the perfect insulation company to take care of your roof rafter insulation for you. 

How To Insulate Between Roof Rafters: Conclusion

Insulating between roof rafters can make a huge difference in reducing your home’s carbon footprint and making your energy bills more bearable. By insulating this often-neglected space, you’ll prevent heat loss, noise, and moisture buildup and even improve your home’s value. 

So you’re wondering how to insulate between roof rafters? Before you get started, take your time to evaluate your attic and choose the proper insulation material. Consider factors such as the R-value, budget, moisture resistance, and fire safety. 

After that, take your measurements precisely and cut the insulation material to fit snugly between your rafters to ensure there are no gaps or leaks.

Finally, pay attention to cases where you shouldn’t insulate between rafters, such as in ventilated attics and flat roofs.

On This Page
Get Your Insulation Quote